1. Field of the Invention
The present invention relates to an elastic boundary wave apparatus in which electrodes are arranged between a plurality of media and, more particularly, relates to an elastic boundary wave apparatus in which a circuit includes a filter, a resonator, and an electrostatic capacitor, and the resonator and the electrostatic capacitor are connected to the filter.
2. Description of the Related Art
In elastic boundary wave apparatuses, IDTs (interdigital transducers) are arranged at the interface of different media, and an elastic boundary wave propagates along the above-mentioned interface. Therefore, when compared with an elastic surface wave apparatus, the elastic boundary wave apparatus does not require a complex package, the construction can be simplified, and a low profile can be achieved. An example of this type of elastic boundary wave apparatus has been disclosed in Japanese Unexamined Patent Application Publication No. 58-30217 described below.
In the elastic boundary wave apparatus, also, similarly to the elastic surface wave apparatus, various resonators, filters, or a combined circuit including a resonator and a filter can be constructed by forming various electrodes.
In the elastic surface wave apparatus, when an electrostatic capacitor is connected to a filter, a comb-shaped capacitor electrode having an electrode configuration similar to that of an IDT is often used. In the elastic boundary wave apparatus, also, in order to improve the amount of attenuation in an elastic boundary wave filter, it is considered that the amount of attenuation can be improved by arranging this type of comb-shaped capacitor electrode at the interface and by connecting it to the filter.
A comb-shaped capacitor electrode has a plurality of electrode fingers that interdigitate with each other in the same manner as in an IDT. Therefore, in a comb-shaped capacitor electrode for simply obtaining electrostatic capacitance, when electrical potential is applied between an electrode finger that is to be connected to one of the electrical potentials and an electrode finger that is to be connected to the other electrical potential, the comb-shaped capacitor electrode occasionally work as an elastic boundary wave resonator. That is, in the comb-shaped capacitor electrode, an undesired elastic boundary wave is excited, and there is a risk that the wave appears as an unwanted resonance.
When the comb-shaped capacitor electrode works as an IDT in the manner described above and an undesired resonance occurs, filter characteristics are deteriorated in a filter apparatus using an elastic boundary wave. In particular, in the frequency characteristics of the comb-shaped capacitor electrode, many responses appear in a frequency band higher than a frequency of a main resonance in the comb-shaped capacitor electrode. Therefore, a main resonance by excitation of the comb-shaped capacitor electrode and many responses at high frequencies appear as spurious responses that are not desired in the elastic boundary wave filter apparatus, and filter characteristics are apt to be greatly deteriorated.